Make documentation of AbstractPlugin useful.

The current documentation - ExtJS 4.0.2 - just states obvious, but not useful, things about plugins. What I would request is that the documentation does the following:

- What is a plugin in ExtJS. The word plugin is used throughout programming and beyond, so it needs to be stated what a plugin is in the ExtJS context.
- What the difference between a plugin and a mixin. When should I choose to implement a plugin and when would I rather choose to implement a mixin.
- Example code presented in a tutorial like style, i.e. showing how to implement a plugin with comments, explanations and considerations.
- It should also be stated that a plugin makes some assumptions about where it is used (I often meet the misconception that a plugin can be used in any class, which is not the case).

Provide turorial on when to implement a mixin instead of doing something like inherit

Mixins are new to ExtJS 4. A tutorial on when and why you should consider to implement a mixin is needed. The tutorial should provide information on the differences between a normal class (one that you would 'requires:' or 'uses:') and 'mixins:'.

Documentation Suggestion: Store Filtering

The documentation for filtering a store provides examples based on passing in filter configs... but lacks the demonstration of passing in a pre-instantiated filter...

The reason I think this is important is because when you pass in a filter config - AbstractStore::decodeFilters() applies the root:'data' property to the config for you... When one of the filters you're applying to the store is already instantiated, this isn't done for you - so it goes without saying - the filter wont work as expected!

I don't think its something that necessary requires a large gravity of attention, but if you could add to the examples (within the store documentation) an example of adding an instantiated filter - I think it may help folks that run into the problem like I did